Square top jib

Discussion in 'Sailboats' started by ned, May 31, 2010.

  1. ned
    Joined: Apr 2010
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    ned Junior Member

    Hi
    Im doing a sience project on trying to design and build a square top jib for a small modle yacht then if this works ill try it out on a 14ft cat. does any body have any experiane in this area?
     
  2. Doug Lord
    Joined: May 2009
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    Doug Lord Flight Ready

    Yeah, I do. This one is about a 20' luff with the head about 1ft.. In the future I'd make the head bigger by about 35-50%.
    The head is supported by a "gaff" that is made of a carbon tube. The "peak" or upper clew is attached to a slide so that it can be adjusted fore and aft like a "mormal" outhaul. It was assumed that this adjustment would be made based on the days' conditions. More sophisticated versions could have the thing adjustable from below but I don't think its necessary.
    The "throat" or upper tack on this sail was a luff wire attached to the "gaff".
    The halyard was attached at about 30% of the length of the gaff aft of the wire-and its position was adjustable. Playing with this has a big effect on the gust response of the sail.
    I had, again , assumed that the very tall, narrow planform of this sail would require massive halyard tension but testing seemed to show that not to be the case. For the initial testing the rig had a backstay but future versions will not. The "peaked up" design of the sails is not to look pretty-it is aerodynamically better than a horizontal square top.....
    You might be interested to know that Tom Speer called the USA wing a "sloop rig". That would make the forward element a full length square top jib.....
    ----
    Featured:
    --excellent gust response which was adjustable on both sails
    -- Because of the way the upper part of the jib was attached the leading edge of the jib tended to slightly over-rotate which allowed the whole rig-when viewed from above-to have a slight twist. This was in addition to the slight twist induced in each sail. This seems to be a benefit-at least thats what I've been told by a couple of people who should know.
    ---------------------------
    On a couple of the images click on the image and then again on the resulting image for largest size. Doesn't work on every image:
     

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  3. CT 249
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    CT 249 Senior Member

    It was tried in International Canoes, by a guy who had been top 5 or 6 in the world. He built a square top jib and supported the leach by a wire from the upper corner of the leach to the hounds.

    When he actually had the chance to sail against a comparable boat to do proper boat-on-boat testing, he found that it didn't work and he gave it up.

    Going back a while, Australian Cherubs sometimes tried a jib with a distinct roach in the head, but they seem to have given it away now.
     
  4. Doug Lord
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    Doug Lord Flight Ready

    =================
    Huh? Sounds like it couldn't possibly work-any pictures? Sounds like he gave up waaay too soon if thats the way he did it....
     
  5. ned
    Joined: Apr 2010
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    ned Junior Member

    hi
    i am looking at some thing more like this
     

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  6. Munter
    Joined: Jul 2007
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    Munter Amateur

    Have you considered tacking?
     
  7. Petros
    Joined: Oct 2007
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    Petros Senior Member

    You could make this work without any battens, you run a sheet from the top corner as well as from the lower one. You could join the two sheets about 2/3rds of the way to where you cleat it, so you only have to handle one at the deck level. The outline of it would look kind of like a large genoa, but with the large triangle of sail cloth from the trailing edge of your actual jib to where the top sheet and the lower sheet meet is removed. This would allow you to roller furl it, and to tack without having any battens or frames to have to clear the mast.

    I hope this makes sense. You essentially use a top edge sheet as well as a lower edge sheet to create trapezoid shaped jib (a truncated triangle), instead of a triangle one. You would have to cleat it near the rear transom to get the correct shape I am thinking of, you will not quite get a square top jib, but fairly close. The top edge would slope downward, inline with the top sheet.
     
  8. ned
    Joined: Apr 2010
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    ned Junior Member

    i was looking at having just stif material up the top and quite fleible bations.
     
  9. gggGuest
    Joined: Feb 2005
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    gggGuest ...

    You see wideish (3 or 4 inches not feet) jib heads quite often in the UK at the moment built up with reinforcement. There are a lot of practical issues which tend to militate against something with a spar or whatever, most notably the effect on mast bend, because either you end up with a very high jib hoist, which is bad for bend characteristics or else a rather low aspect ratio jib which rather defeats the object.
     
  10. Doug Lord
    Joined: May 2009
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    Doug Lord Flight Ready

    ====================
    Seems to me the main can be designed for the bend characteristics of the mast-especially when the gain may be great for a full span squaretop jib..
     
  11. Chris Ostlind

    Chris Ostlind Previous Member

    The main always has to be designed for the bend potential of the mast.

    The comment 3g is putting out there is that if you have the jib being hoisted to the mast head, as shown, then you will be doing a number on the mast bend characteristics for a potentially dubious goose chase that amounts to looking cool and nothing else.

    After looking at those pictures of the aeroskiff, just how did you effect anything like a tunable rig on that boat. The way the gantry thingy is rigged, it looks like the outcome of cranking on the backstays was turning the hull into a banana.
     
  12. Doug Lord
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    Doug Lord Flight Ready

    The backstay on the original boat was unnecessary. Using a properly designed
    and set up squaretop jib WITH a square top main has many benefits and is
    completely adjustable. The rig has to be designed for this application and seems to work quite well:
    1) excellent gust response from both sails and the rig as a whole,
    2) easily adjustable for any condition,
    3) adjustable over-rotating jib adds slight twist to whole rig while twist in each sail is individually adjustable,
    4) jib halyard tensions leech and luff in a completely adjustable manner,
    -- attachment position of the halyard to the micro-gaff is variable fore and aft,
    5) in the ideal incarnation of the rig the main luff(or an extension thereof) comes down to the deck further extending the jib slot as compared to "normal" sloop rigs. Even more effective than a "normal" masthead rig because of the square tops on both sails(and because of their characteristics).
    6) adjustable upper outhaul at the "peak" of the micro-gaff on the head of the jib.
    7) newer incarnation will allow upper outhaul adjustment on the main as well.
    -- the adjustable upper outhaul has been tested on two full size boats and for years on several different models.
    8) the finished version of the rig- on a boat that can capsize- uses a flotation pod/endplate first tested 30 years ago on a model.
    9) the main can use a luff sock with camber inducers-and can be reefable.
    ----------------
     
  13. ned
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    ned Junior Member

    i have come up with this plan?
     

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  14. Doug Lord
    Joined: May 2009
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    Doug Lord Flight Ready

    ================
    Try it on a model rig-and see my post above for the things you might want to look for......
     

  15. ned
    Joined: Apr 2010
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    ned Junior Member

    yea ive got a small 60cm keeler and a swimming pool
     
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